The idea of building forts stemmed from an idea where players had to play mini-games in order to complete their fortifications. They later changed it so that the forts built themselves whenever the player opened up the "edit" function.
Contributed by GamerBen144
Fortnite was stuck in development hell, with such development beginning as early as 2011 when it was first announced at the Spike Video Game Awards. The earliest idea for Fortnite was a horror-style third-person shooter similar to Gears of War, another series developed by Epic Games and later sold off to Microsoft.

Fortnite was originally made using Unreal Engine 3, but later shifted to its successor Unreal Engine 4 due to new high-tech functions that its preceding engine lacked. Because of this, the game was almost a PC-exclusive game and was set for a 2013 release, as Epic Games did not want to wait months for a game to get approved by console manufacturers, such as Nintendo and Sony, although they later acknowledged that a console release could not be ruled out. As Epic shifted to Unreal Engine 4 however, they immediately ran into development problems, which resulted in them hiring MMORPG developers to help finish the game.
Contributed by GamerBen144
The idea for a "Battle Royale" mode stemmed from battle-royale type games including PlayerUnknown's Battlegrounds, a game that had been released halfway through Fortnite's development. The staff at Epic Games loved these games so much that they decided to take the formula and make it a separate mode within the game.
Contributed by GamerBen144